Alkylated aromatic compounds obtained by alkylating aromatic compounds are commercially important intermediate stock materials. For example, cumene obtained by reacting benzene with propylene is a very important compound as a reactant for the synthesis of phenol.
Cumene is generally produced by alkylating benzene with propylene. Numerous studies have been made on this alkylation process. For example, Japanese Patent Application Kokai No. 40419/1982 discloses alkylation under liquid phase conditions using Lewis acid catalysts. U.S. Pat. No. 2,375,724 discloses the use of solid phosphoric acid catalysts.
Cumene is also produced by alkylating benzene with isopropanol. Reaction under gas phase conditions using solid acids is well known as disclosed in Japanese Patent Application Kokai No. 159427/1983 and 159430/1983. Reaction under liquid phase conditions has also been proposed. For example, U.S. Pat. No. 4,011,278 discloses alkylation in the presence of an H-mordenite catalyst having a silica-to-alumina ratio adjusted to 40.
These processes for the synthesis of cumene have several problems. For example, the alkylation using Lewis acids needs the presence of alcohols because the catalysts are readily deactivated by water in the reaction system, and alkylation with alcohols is impossible. The gas phase alkylation with isopropanol is substantially impractical because of low alkylation conversion and quick deterioration of the catalyst which must be frequently regenerated to compensate for such a short lifetime. The liquid phase alkylation with isopropanol is high in alkylation conversion, but is doubtful about its practical effectiveness because the percent yield of the end product is not demonstrated. This process has left a problem to be overcome for commercial production because expensive catalysts such as ZSM-5 zeolite having a high silica-to-alumina ratio and dealuminated H-mordenite are used for water repellency.
It is well known in the prior art to react benzene with propylene to produce cumene, to oxidize cumene to produce cumene hydroperoxide, and to acid cleave cumene hydroperoxide into phenol and acetone. A conventional phenol preparing process having these steps combined is generally known as the cumene or cumol-phenol process.
It is also an old well-known technique to hydrogenate acetone into isopropanol. This technique is still widely used at present for the assay of the catalytic activity of a hydrogenating catalyst and other purposes. For example, the activity of Raney nickel catalysts is often examined by comparing their acetone hydrogenating ability. Several advanced processes have been proposed as disclosed in Japanese Patent Application Kokai Nos. 12729/1987 and 7338/1987.
For the alkylation of benzene with isopropanol, reaction under gas phase conditions using solid acids is well known as disclosed in Japanese Patent Application Kokai Nos. 159427/1983 and 159430/1983. Reaction under liquid phase conditions is also proposed as disclosed in U.S. Pat. No. 4,011,278.
As to the reuse of the acetone which is by-produced in the cumene process, for example, by its conversion into isopropanol, no useful proposals have been made.
The phenol preparing process generally known as the cumene process is characterized by the production of acetone by-product, which is advantageous from some aspects, but disadvantageous from other aspects. More particularly, it is an advantage in that simultaneous production of two products in a single preparation unit is more efficient than individual production in separate units. In turn, if the proportion of phenol and acetone produced is unbalanced relative to their commercial demands, one for less demand is produced largely in vain.
As is known in the art, acetone is available in surplus for these years. Thus the production of acetone by-product is now considered as a serious drawback of the cumene process. Although acetone has found the majority of its application as a starting material for preparing methyl methacrylate, the demand for acetone is decreasing because of a switch of the starting material for preparing methyl methacrylate to another.
Under the circumstances, there is a need for the development of a phenol preparing process which is devoid of production of acetone and other by-products. Although several proposals have been made, there is available no process capable of preparing phenol in satisfactory yields.
In addition, impurities in propylene often cause a drawback during the preparing step of cumene from benzene and propylene in the cumene process for the preparation of phenol. That is, propylene for use in the cumene preparation is manufactured generally using crude oil as the starting material. Crude oil, however, contains sulfur compounds and various heavy metals, and these impurities are sometimes carried in propylene as trace contaminants during its manufacturing process. For example, carbonyl sulfide (COS) as a sulfur compound or As as a heavy metal contaminant in propylene inhibits function of a catalyst (aluminum chloride-HCl complex) for use in the cumene preparation, thus disturbing normal progress of the cumene synthesis. Therefore, a strict purification process is perproduced to avoid contamination of propylene with these impurities. Types and quantity of these contaminants, however, vary depending on not only the crude oil source but also the difference in the process conditions for the preparation of propylene from crude oil. Such irregularity burdens the propylene purification process with exceptionally complex and severe steps.
In consequence, a process for the preparation of propylene with highly stable purity containing no such impurities has been expected to be established, for the purpose of reducing the extreme burden of steps in the propylene purification process.
As to the alkylation of aromatic compounds, a variety of processes are known in the art. For example, the alkylating process using olefins has been widely used in the industry, becoming one of the important industrial techniques. Various proposals have also been made on the alkylating process using alcohols.
Several prior art techniques for the alkylation of aromatic compounds using alcohols are described below. (1) Japanese Patent Application Kokai No. 159430/1983 discloses a process for preparing a 1,4-dialkylbenzene compound in the presence of an oxide-modified zeolite catalyst.
(2) Japanese Patent Application Kokai No. 263934/1986 discloses a process for preparing p-xylene by alkylating toluene with methanol in the presence of a ZSM-5 type zeolite catalyst.
(3) Japanese Patent Application Kokai No. 216128/1983 and 159427/1983 disclose processes for preparing a monoalkylbenzene or dialkylbenzene by reacting benzene or alkylbenzene with an alcohol in the presence of a proton-exchanged mordenite type zeolite catalyst.
(4) U.S. Pat. No. 4,011,278 discloses a process for alkylating various aromatic compounds with alcohols in the presence of a ZSM-5 type zeolite catalyst.
All these processes intend to increase the percent yield of the end alkyl aromatic compound by improving the catalyst.
It is generally difficult for the alkylation of aromatic compounds to selectively produce a desired compound. More particularly, alkylated products are usually more reactive to the alkylation than starting reactants so that further alkylation may proceed to form higher alkylated products, providing a bar against the selective alkylation. For this reason, a number of proposals have been made to achieve selective alkylation by improving the catalyst.
In the alkylation of aromatic compounds, the reactivity depends on the acidic nature and shape of the catalyst used. It is generally believed that the acidic nature of catalyst controls the reaction rate and the catalyst shape controls the selectivity.
It is to be noted that alkylation using alcohols yields water. In general, catalysts having acidic nature weaken their acidity in the presence of water. Even a loss of catalytic activity can occur particularly when the catalysts used are solid acids. Thus in the event that water can form, various measures have been taken including the use of severe reaction conditions for increased reactivity and the use of water-resistant catalysts.
However, the selectivity of the desired compound is reduced under such severe reaction conditions or in the presence of improved catalysts, for example, modified strongly acidic catalysts, because highly alkylated products are formed or dehydration of alcohols results in olefin by-products.
For the alkylation of aromatic compounds using alcohols as the alkylating agent as described above, no commercial technique capable of selective production of a desired reaction product has been completed.